Several techniques are used to detect defects possibly present in an electronic integrated circuit. One of these techniques, called laser voltage imaging (LVI), analyzes a laser beam reflected by the circuit. To do this, the surface of the circuit is swept by a laser beam while the circuit is powered with a continuous signal and while a periodic test signal is delivered to the data input of the circuit. The reflected laser beam is then measured and delivered, on the one hand, to an imager capable of producing a map of the surface of the circuit using the intensity of the reflected laser beam, and, on the other hand, to a spectrum analyzer performing a frequency analysis targeted on the frequency of the test signal, allowing the frequencies present to be mapped onto the surface of the circuit and possible frequency signatures to be detected.
The frequency signatures correspond to an intensity peak for a given frequency, in this case the test frequency, in the reflected laser beam.
The imager produces an image using the variation in intensity of the reflected laser beam. The image thus produced comprises dark regions and lighter regions making it possible to discern, in the image, the location of the various electronic components. The spectrum analyzer only measures the intensity of the reflected laser beam at the frequency of the test signal and thus determines in which locations on the electronic integrated circuit the maximum intensity was reflected at the frequency of the test signal.
This known technique thus allows specific available frequencies to be mapped onto active regions of the electronic integrated circuit. It uses the properties of the materials, and more particularly the variation in the charge density within the materials.
However, employing LVI on its own does not allow the defective electronic components to be reliably and precisely located. This is because, LVI employs a periodic test signal and a spectrum analyzer centered on the frequency of the test signal. This configuration results in a frequency-signature map corresponding both to defective electronic components and to functioning switches or transistors. This is because functioning transistors show charge densities modulated at the frequency of the test signal, and the transistors also heat up at this same frequency. Consequently, the spectrum analyzer centered on the test frequency outputs a signature frequency both for functioning transistors, which heat up or modulate charge density at the test frequency, and for defective electronic components, which heat permanently. It is therefore not possible to differentiate between the frequency signatures of transistors that are functioning normally and the frequency signatures of defective components.